One third of the sum is paid before of the grand day and the rest within 20 days of delivery. You may pay using PayPal or bank transaction.
Delayed payment means that an interest of 12 % will be added, counting from the date of the invoice. A delayed payment is followed by a formal reminder and a few extra days to pay the agreed amount plus interest. Further delayment after that is object for legal enforcement with the support from the authorities in your country.